Light emitting diodes are frequently used to present an alphanumeric display. When used in that fashion, a separate mosaic or matrix of light emitting diodes is typically used for each character. Light emitting diode patterns, or mosaic patterns, are frequently called a "matrix". The term matrix usually refers to a row-column arrangement, and it is not intended that the pattern should be limited to such a row-column configuration. When it is desired to present a picture on a large mosaic made of sectors of light emitting diode mosaics, it is necessary that the rows and columns of adjacent sectors be aligned and that the separation between diodes on adjacent sectors be the same as the separation between diodes within the sectors.
Existing diode mosaic sectors which are designed to present a picture have a diode spacing of about 32 to the inch. When it is attempted to create such mosaics with densities of 64 or 128 per inch to obtain better resolution, the driving of the rows and columns of the individual mosaic sectors becomes difficult or impossible.
Present light emitting diode matrices are mounted upon ceramic substrates which are costly to produce and prone to cracking.
Typically the mosaic is connected to a printed circuit board upon the surface of the ceramic substrate opposite the diode. Metalization of the circuit is typically either a gold ink or plated circuits which are restricted to a low thickness of, for example, 0.25 to 0.5 mil. Such circuits are preferably of gold which is costly both because of the price of gold and because the process used requires many steps.
Conventional light emitting diode mosaics have their anode contact on the outer surface and either total coverage of the rear cathode contacts or a large random dot pattern to permit silver epoxy mounting and wire bonding to the anode contact. The anode is typically in the center of the light emitting diode, and it obscures a large portion of the emitting light.
In the prior art it is customary to form monolithic chips of light emitting diodes, and then to sawcut or abrade the surface between the diodes to isolate the junctions.
It is customary to use a conductive epoxy to mount the light emitting diodes on the substrate. The anodes of the diodes are then stitch-bonded.
The conductors connecting the anodes are reflective, and a circular polarizer filter needs to be used to prevent scintillation. Such a filter typically has a transmittance of only about 25% which requires the diodes to be brighter in order to be seen in the presence of sun light. With a brighter light required, the diodes must dissipate more heat.
The diodes are typically mounted with a low density because of the requirement that the mosaic sectors be mounted in juxtaposition with the pitch between the outside rows and columns of adjacent mosaic sectors being the same as the column and row spacing between adjacent diodes within the mosaic.
It is usual to mount the row and column driver chips on the back of the substrate or between separate characters on the front of the substrate.
To avoid these enumerated problems and others, the apparatus of this invention has been conceived.